This invention relates to electrostatic copiers that produce an image by depositing a dry, powdered toner on paper. The invention relates specifically to a device for pressure fusing the toner image to the paper.
In most copying machines, the copying machine forms an optical image of the material to be copied which is projected onto the surface of a sensitized semi-conductor, forming an electrostatic image. A dark color, electrostatically sensitive, powdered material, called toner, is brought into contact with the sensitized semi-conductor. Toner particles adhere to the areas on the semi-conductor where an electrostatic charge is present. The toner particles, maintaining the pattern formed on the semi-conductor, then are transferred to a sheet of paper. The toner particles are bonded to the paper, forming a permanent reproduction of the material being copied.
Three basic methods have been used to bond the toner to the paper: solvent fusing, thermal fusing and pressure fusing. In addition a combination of heat and pressure have been found to be effective. Solvent fusing is least desirable because personnel can be exposed to harmful solvent vapors. Thermal fusing, or the combination of heat and pressure to fuse the toner, is the most commonly used technique. However, with any thermal based operation, power consumption and warm-up time are significant. Pressure only fusing has not been used extensively because there have been no good pressure fixable toners that will operate with plain paper at pressures less than 200 to 300 pounds per linear inch (pli). At these pressures, the size, weight and cost of the pressure applying elements become significant factors in a copying machine.
In prior art electrostatic copiers that use pressure fusing devices to fix or fuse the toner image to paper, the paper with an unfused image of loosely adhereing toner particles is passed between two parallel rolls that are pressed together. Generally, the rolls are supported at their ends on bearings. Most often, only one of the rolls is driven, the second roll being an idler roll. Usually the rolls are of equal diameter and, hence, are driven at equal surface velocities. In order to achieve essentially uniform fusing pressures along their entire length, the rolls are of relatively large diameter so that their deflection can be low.